Power line communications

ABSTRACT

An apparatus for coupling signals to a transmission line, such as a power line ( 120 ). The apparatus comprises an input for receiving a wanted signal; a first coupler for coupling the wanted signal (V main ) onto the line at a first position (Y); a cancelling means (W B , W R , W Y ), coupled to the input, for deriving a cancelling signal (V aux ) from the wanted signal; a second coupler for coupling the cancelling signal onto the line at a second position (X), spaced from the first position; wherein the wanted signal and the cancelling signal destructively combine in a single direction of propagation along the line while enabling the wanted signal to propagate in the other direction along the line. The apparatus can be used to minimise radiation from an unshielded part of an electricity distribution network, such as substation ( 100 ), or to allow re-use of a frequency band on other power lines.

TECHNICAL FIELD

This invention relates to an apparatus and method for coupling signalsonto a transmission line. It has particular application to couplingradio frequency (RF) signals onto an electricity distribution networkwhich is used to transport telecommunications signals.

BACKGROUND TO THE INVENTION

It is known to transport telecommunications signals over an electricitydistribution or power transmission network. Patent Application WO94/09572 A1 (NORWEB) describes such a network. Delivering atelecommunications service in this manner is attractive as it overcomesone of the greatest costs in providing a new telecommunications networki.e. installing cabling to each subscriber. Existing electricitydistribution cabling is used to carry the telecommunications signals.

FIG. 1 shows an electricity distribution network which is adapted tocarry telecommunications signals. Mains electricity enters the networkfrom an 11 kV transmission line 105 and is transformed by substation 100into a 415V supply which is delivered over cable 120 to subscribers S. Abase station BS couples telecommunications signals V_(B), such as voiceand data signals, at injection point 110 onto distribution cable 120.The telecommunications signals propagate over the cable on radiofrequency carriers to transceiver units TRX at subscriber premises S.

One of the problems with transporting RF signals over the electricitydistribution network is that of unwanted radiation of RF energy. Thedistribution network was not designed to carry RF signals.

Electricity distribution cables, such as cable 120, have a concentricstructure. The inner section of the cable comprises groups of conductorswhich carry one or more of the three supply phases. This inner sectionis surrounded by an outer section which is coupled to earth. Thesecables have similar screening properties to coaxial cables, andconveniently this screening, coupled with the underground burial of thecables, is effective at the radio frequencies (RF) that are used fortransporting telecommunications signals.

The internal wiring at subscribers' premises S is unscreened, and couldpotentially cause radiation problems. However, by filtering off the RFsignals at the point where the electricity feeder cable becomesunscreened radiation of RF signals is minimised.

The other significant point where radiation can occur is at thesubstations 100 where electricity is transformed from 11 kV to 415V.Substations have busbars which are typically mounted as a grid array onthe substation wall. The busbars are shielded from view but frequentlyare electrically unscreened. This is because screening is consideredunnecessary at the 50 Hz mains frequency. At RF frequencies the busbararray functions as an antenna, radiating the RF signals which itreceives via the distribution cables into the surrounding area. This isa undesirable as it causes interference with equipment operating atthese frequencies. This radiation may also violate regulations onElectromagnetic Compatibility (EMC).

One of the solutions to minimise radiation from the busbars is to screenthe busbar array, or to screen the entire substation building. Somemodern substations are equipped with metal casing around the busbars.However, the majority of substations are unscreened brick structures. Itis undesirable to renovate all of these structures to improve theirscreening as it increases the cost of providing a telecommunicationsservice over the network.

An alternative solution to the radiation problem is to restrict thepower at which the RF signals are transmitted over the network, suchthat radiation occurring at substations falls below acceptable limits.This causes problems with subscribers' equipment, particularly to thosesubscribers furthest from the point at which RF signals are injectedonto the network. Subscriber equipment needs an acceptable signal tointerference ratio in order to detect the wanted RF signals. Withconsiderable interference on the network, this demands a reasonably hightransmit power.

The problem of radiation at the substation is compounded by the factthat RF signals are usually injected onto the distribution networkadjacent to the substation. The reason for injecting at this point isbecause one base station can easily be coupled to each of a group of415V cables (120, 130, 140 in FIG. 1) which all converge at thesubstation.

A paper entitled “Adaptive Interference Cancellation for Power LineCarrier Communication Systems” at pp. 49-61 of IEEE Transactions onPower Delivery, Vol 6, No 1, January 1991, addresses the problem offrequency reuse in a power line carder system. A portion of atransmitted signal or a first line section which leaks through a linetrap onto a second line section is cancelled by applying a cancellingsignal to the second line section.

DE 2 523 090 describes a directional signal generator which controllablypropagates in ore direction along a line. This uses an attenuatornetwork in series with the line.

SUMMARY OF THE INVENTION

The present invention seeks to minimise the above problem.

According to a first aspect of the present invention there is providedan apparatus for coupling signals to a line, the apparatus comprising:

an input for receiving a wanted signal;

a first means for coupling the wanted signal onto the line at a firstposition;

a cancelling means, coupled to the input, for deriving a cancellingsignal from the wanted signal, the cancelling means being operable tophase-shift the wanted signal,

a second means for coupling the cancelling signal onto the line at asecond position, spaced from the first position;

and wherein the apparatus is arranged so that the combination of thephase-shift and propagation delays experienced by the signals causes thewanted signal and the cancelling signal to destructively combine in asingle direction of propagation along the line while enabling the wantedsignal to propagate in the other direction along the line.

Preferably the cancelling means comprises a weight which is operable toscale the wanted signal in amplitude.

Preferably the spacing of the first and second couplers is substantiallyone quarter of a wavelength of the wanted signal. This maximises signalpower in the wanted direction of propagation.

Preferably the apparatus also has a monitor for sensing the combinationof the wanted and cancelling signals at a position on the line andfeeding the sensed signal to a calculating means which controls thecancelling means. This allows a more effective cancellation.

The calculating means can perform an iterative technique in whichperturbations are applied to the value of the weights and the sensedsignal is monitored to determine the effect of the perturbations.

The calculating means may alternatively perform an iterative techniquein which the sensed signal is correlated with a portion of the wantedsignal to determine updated weight values.

Preferably the apparatus is used to couple telecommunications signals toa power line such as a distribution line of an electricity distributionnetwork for serving a plurality of subscribers.

In the situation where the telecommunications signals are coupled ontothe electricity distribution network at a position between an unshieldedpart of the network and the subscribers, and the wanted and cancellingsignals destructively combine in the direction of the unshielded part,this prevents unwanted radiation of signals from the unshielded part ofthe network.

A further application of controlling the direction of propagation ofsignals along the line is in allowing a particular frequency band whichis in use on one line to be reused on the other lines. This has aparticular use where an electricity distribution network has severaldistribution lines served by a common substation.

A further aspect of the invention provides a method of coupling signalsto a line, the method comprising:

receiving a wanted signal at an input;

coupling the wanted signal onto the line at a first position;

deriving, at a cancelling means, which is coupled to the input, acancelling signal from the wanted signal, the cancelling means beingoperable to phase-shift the wanted signal;

coupling the cancelling signal onto the line at a second position.spaced from the first position;

and wherein the combination of the phase-shift and propagation delaysexperienced by the signals is arranged such that the wanted signal andthe cancelling signal destructively combine in a single direction ofpropagation along the line while enabling propagation of the wantedsignal along the line in the other direction.

A further aspect of the invention provides a method of couplingcommunications signals on to an electricity distribution networkcomprising a substation serving a plurality of distribution lines, themethod comprising:

coupling communications signals occupying a frequency band to one of thelines by coupling a wanted communications signal onto the line at afirst position and coupling a cancelling signal onto a line at a secondposition, spaced from the first position such that the wantedcommunications signal and cancelling signal destructively combine in adirection of propagation towards the substation; and,

reusing the frequency band for coupling different communications signalson to another one of the plurality of lines.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show by way ofexample how it may be carried into effect, embodiments will now bedescribed with reference to the accompanying drawings, in which:

FIG. 1 shows an electricity distribution network which is adapted totransport telecommunications signals;

FIG. 2 shows part of the network of FIG. 1 in more detail;

FIG. 3 shows an, arrangement to minimise radiation from an unscreenedpart of the network of FIGS. 1 and 2;

FIG. 4 describes the operation of the arrangement of FIG. 3;

FIG. 5 shows the arrangement of FIG. 3 modified to include a monitoringcircuit;

FIG. 6 shows one form of calculating circuit for use in the arrangementof FIG. 5;

FIG. 7 shows an alternative form of calculating circuit for use in thearrangement of FIG. 5;

FIG. 8 shows an improvement to the arrangement of FIG. 5;

FIG. 9 shows frequency reuse in an electricity distribution network.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring again to FIG. 1, this shows an electricity distributionnetwork which transports telecommunications signals. FIG. 2 shows partof this network in more detail. Distribution cable 120 has threeseparate phase lines: Blue, Red and Yellow. Each of the phase lines arecoupled to a respective busbar in substation 100. The output ofbasestation BS is coupled to a three-way splitter 210. Each of the threeoutput lines is coupled, via a high-pass filter unit 200 to a respectivephase line of cable 120. The mains filter serves to pass only signals inthe RF bands which are used for transmission of telecommunicationssignals and to block the flow of mains electricity into the basestation.Telecommunications signals propagate along cable 120 in two directions;towards substation 100 and towards subscribers. Signal V_(B) is shown onthe blue phase line. Telecommunications signals reaching substation 100radiate RF energy 220.

FIG. 3 shows the same section of the network as that shown in FIG. 2,but with modifications to minimise propagation of RF signals along cable120 towards substation 100.

The output of basestation BS is split, as before, into a feed for eachphase line. The feed for each phase line is split into two components; amain component V_(main), and a cancelling component V_(aux). V_(main)couples to cable 120 at position Y via a high-pass filter 200, asbefore. V_(aux) is coupled to cable 120 at a position X, spaced fromposition Y by a distance d. V_(aux) is weighted by a weight W_(B) whichphase-shifts the signal, and can also scale the signal in amplitude.V_(aux) and V_(main) are related in phase such that at point X, and inthe direction towards substation 100, V_(aux) and V_(main) are offset inphase by 180° i.e. they are in anti-phase and destructively cancel.Therefore, the section of cable between point X and the busbars carriesno RF signals, or RF signals at much reduced levels. Point X could belocated at the busbars themselves.

Joints at points X and Y should be made using the same jointingtechnique, such that the RF coupling characteristics track in amplitudeand phase. This gives optimum broadband cancelling performance.

By appropriate spacing of the main and auxiliary signal feeds, andappropriate phase-shifting at the weight, a further advantage can begained. V_(main) and V_(aux) can be related in phase such that at pointY (and along the cable towards subscribers) they are in phase i.e. theyconstructively combine.

Typical transmission frequencies are the bands 2-6 MHz and 10-14 MHz.The optimum performance, with cancellation in the direction towards thesubstation and constructive interference in the direction towards thesubscribers is achieved with the feed spacing d=λ/4 and a weightingphase-shift of 180°. The typical level at which the main signal can becoupled onto the line is 1 Vrms. Such a high level may cause radiationproblems in conventional systems.

The operation of the system will now be described further with referenceto FIG. 4. Considering FIG. 4, the delays and phase shifts are arrangedsuch that signals injected from the main feed and the auxiliary feed,and propagating towards the bus bars from point X, destructivelyinterfere, but the signals propagating along cable 120 away from the busbars do not destructively interfere. By arranging distance d to beapproximately d=λ/4, the signals propagating in the direction towardsthe subscribers will be maximised. Cancellation at point X can beachieved for any distance d. At one extreme, it is possible to reducethe feed separation distance so that both feeds can be coupled to thecable within a distance which is small enough to fit within a section ofcable exposed by the digging of a single hole.

Let us consider the main and auxiliary paths from the signal input topoint X. When passing through the main path, signals undergo delay τ₁,in the feed cable, and delay τ₂ propagating through distance d to pointX. The auxiliary path is arranged such that the feed delay to point X isequal to τ₁+τ₂. By setting the weight θ weight to give a 180° phaseshift, it can be seen that ideally broadband cancellation between thesignals propagating along the two paths to X can be achieved. Inpractice the weight value in phase and amplitude can be adjusted by anadaptive loop to compensate for mismatches between the feed paths.Alternatively, a fixed phase shift of 180° and zero attenuation can beset.

Now let us consider the main and auxiliary paths to point Y. Signalspropagating along the main feed are delayed by τ₁. Signals propagatingthrough the auxiliary path are delayed by τ₁+τ₂ to point X, and by afurther τ₂ when propagating through distance d to point Y, giving atotal delay of τ₁+2τ₂. In addition, the signals propagating through theauxiliary feed are phase shifted by 180°. At point Y, therefore, the twocomponents of the input signals are combined, one of which is delayed byτ₁, and a second component which is delayed by τ₁+2τ₂, and also phaseshifted by 180°. The magnitude of the resultant signal will depend onthe carrier frequency, with the maximum amplitude occurring when thedelay 2τ₂ produces a phase shift of 180°, such that when combined withthe phase shift through the weight a total phase shift of 360° isobtained giving constructive interference. Constructive interferenceoccurs when the phase difference is 0° or a multiple number of wholewavelengths. This requires a separation distance of d=λ/4. Hence optimumtransmission in the desired direction will be obtained with separationof d=λ/4, but useful performance will still be obtainable with otherseparations, except in the extreme case with zero separation.

FIG. 5 shows the arrangement of FIG. 3, further modified so that it ispossible to monitor the success of the cancellation process.

Sensing coils 600, 610, 620 are arranged, one per phase line, to detectsignals present on the busbar. The sensing should preferably be by aninductive coil which is wound around the busbar, or laid closelyadjacent to it. Sensing is also possible by a capacitive link or by oneor more antennas located close to the busbars.

Each sensing coil is coupled to a switch 630 and a weight calculationunit 640. The weight calculation unit outputs control signals to each ofthe weights, W_(B), W_(R), W_(Y). Switch 630 allows a single calculationunit to be time-shared among the monitoring signals. Calculation unit640 operates to apply weight values which minimise the level of themonitoring signal detected by the sensing coils. An attenuated signal isadequate for monitoring, and this allows weakly coupled inductive coilsto be used.

While it is preferable to sense at the unscreened section of the networkwhere radiation is likely to occur, it is also possible to sense atanother point nearer position X, or even at position X itself. However,because cable 120 is reasonably well screened, another form of sensingdevice would be needed, such as a capacitive link.

The operation of the weight calculation unit 640 will now be described.There are two main methods of calculating weight values; by perturbationand by a correlation technique.

FIG. 6 shows a weight cancelling unit which performs the perturbationtechnique. A monitoring signal is applied to a channel filter 700, whichpasses only those frequencies which are of interest (the RF frequencieswhich are to be cancelled). A power detector 710, shown simply as adiode detector D and capacitor C, provide a power measurement which isapplied to an analogue to digital converter 720. The output of the A-Dconverter is fed to a microprocessor 730 which performs a perturbationalgorithm. The microprocessor outputs a set of weight control signalswhich control the in-phase (I) and quadrature (Q) elements of eachweight. The perturbation process works by applying steps in the size ofI and o weight values and monitoring how that affects the cancellation.The algorithm can work by successively changing I up, I down, Q up, Qdown. After these four steps have been performed the change in I or Qwhich had the best effect is adopted. This process continues until thebest cancellation effect is achieved.

The second technique for calculating weight values is by correlation.This is shown in FIG. 7. As with the perturbation technique, an inputfrom the sensing coils is applied to a filter, which passes only theband of RF frequencies which are of interest. The filtered signal issplit, by a coupler 740 into in-phase (I) and quadrature (Q) components.This forms the sum component (S). A portion of the input signal from thebasestation BS is split by splitter 780 and fed into a second coupler750 which also splits the signal into I and Q components. This forms theelement component (E). The two sets of I and Q components are fed to acorrelator 760. A-D converters operate on each of the four inputs to thedevice. The correlator performs a correlation of the E and S componentsand outputs the result to a microprocessor 770 which performs a weightupdate algorithm. A typical algorithm is:

W _((K+1)) =W _(K−μ) E*S

where E*S is the correlation function.

Microprocessor 770 outputs a set of control signals to control the valueof the weights. This technique, similarly to the perturbation technique,is iterative, and is repeated until the best cancellation is achieved.

Once the weight values have been set, the weight calculation processshould only need to be repeated periodically.

Cancellation is most effective at the carrier frequency where the mainand auxiliary signals are exactly in anti-phase. Moving each side ofthis frequency the cancellation effect will be decreasingly lesseffective. This is due to imperfections in the tracking over frequencybetween the phase and amplitude responses of the main and auxiliarychannels and due to mismatch in the delay. Preferably the centrefrequency of the band is chosen as the frequency where cancellation ismost effective, e.g. 4 MHz for the band 2-6 MHz. This scheme istherefore most effective with TDMA systems such as DECT, which employ alimited number of time-shared carriers. A broader band cancellation ispossible by modifying the arrangement as shown in FIG. 8.

In FIG. 8, the signals in the auxiliary path are split into severalportions, which are each delayed by differing amounts and separatelyweighted before recombination. The weights are under the under controlof the adaptive loop, and adapted in turn in a time-shared manner.

It is proposed to use time division multiple access (TDMA) or timedivision duplex (TDD) transmission schemes such as DECT or CT2. This isbecause these schemes use a single carrier for transmission in both theupstream and downstream directions. This simplifies filtering equipmentwhich is needed at the subscriber premises. It is the downstreamtransmissions from the basestation to subscriber premises which causesradiation problems because high power RF signals are injected near tothe substation. Upstream transmission from subscribers to thebasestation arrive at the basestation at low levels which should notcause radiation problems. The subscribers located nearest to thebasestation are controlled such that their transceivers transmit at alower level compared with other transceivers located further from thebase station.

Cancellation is operable during the period when the base station istransmitting downstream. During the basestation receive period, whensubscribers transmit upstream, the auxiliary path is not used and thebasestation receives only via the main path.

Alternatively, the auxiliary path can be utilised in addition to themain path during the receive cycle. In this case, the auxiliary pathneeds to establish the same amplitude and phase response in both receiveand transmit directions.

The effect will be to prevent reception of signals from the direction ofthe bus bars, and to preferentially receive signals originating from thesubscriber oh cable 120.

Transmitting telecommunications signals in one direction along a powerline has two main applications. Firstly, by transmitting only in adirection away from the substation radiation from the substation isminimised. Secondly, by transmitting only in a direction away from thesubstation, it is possible to reuse the same band of frequencies onseveral power lines. FIG. 9 shows an electricity distribution networkwith a substation 100 which serves three distribution cables 120, 130,140. Each distribution cable is served by a respective communicationsbase station BS1, BS2, BS3. The common coupling of cables 120, 130, 140at substation 100 means that telecommunications signals from one line,eg line 120, will flow onto the other lines 130, 140. Couplingtelecommunications signals onto each line so that they propagate awayfrom the substation, and have a negligible component in the directiontowards the substation, allows the same frequency band to be used byeach base station BS1, BS2, BS3. Frequency reuse may be desirable wheretraffic demand from the subscribers on each line requires a base stationto serve just that line, but there is only a limited band of frequenciesavailable for power line communication. The limit on the band offrequencies for power line communications may be due to governmentregulations or because it is found that a particular band of frequenciesoffers optimum performance.

In FIG. 9 signals V₁, V₂, V₃ represent the directional transmissionsfrom base stations BS1, BS2, BS3 which can each share a common frequencyband.

What is claimed is:
 1. Apparatus for coupling signals to a line, theapparatus comprising: an input for receiving a wanted signal; a firstmeans for coupling the wanted signal onto the line at a first position;a cancelling means, coupled to the input, for deriving a cancellingsignal from the wanted signal, the cancelling means being operable tophase-shift the wanted signal; a second means for coupling thecancelling signal onto the line at a second position, spaced from thefirst position and wherein the apparatus is arranged so that thecombination of the phase-shift and propagation delays experienced by thesignals causes the wanted signal and the cancelling signal todestructively combine in a single direction of propagation along theline while enabling the wanted signal to propagate in the otherdirection along the line.
 2. Apparatus according to claim 1 wherein thecancelling means is operable to scale the wanted signal in amplitude. 3.Apparatus according to claim 1 wherein the spacing of the first andsecond couplers is substantially one quarter of a wavelength of thewanted signal.
 4. Apparatus according to claim 1 further comprising amonitor for sensing the combination of the wanted and cancelling signalsat a position on the line and feeding the sensed signal to a calculatingmeans which controls the cancelling means.
 5. Apparatus according toclaim 4 wherein the cancelling means comprises a weight and thecalculating means performs an iterative technique in which perturbationsare applied to the value of the weight and the sensed signal ismonitored to determine the effect of the perturbations.
 6. Apparatusaccording to claim 5 wherein the calculating means performs an iterativetechnique in which the sensed signal is correlated with a portion of thewanted signal to determine updated weight values.
 7. Apparatus accordingto claim 1 wherein the line comprises a power line and the wanted signalcomprises a telecommunications signal.
 8. An electricity distributionnetwork for sensing a plurality of subscribers, the networkincorporating an apparatus according to any one of the preceding claims,and wherein the wanted signal comprises a telecommunication, signalwhich is coupled onto a distribution wire of the network.
 9. Anelectricity distribution network according to claim 8 wherein thetelecommunications signal is coupled onto the network at a positionbetween an unshielded part of the network and the subscribers.
 10. Anelectricity distribution network according to claim 8 wherein thedistribution cable comprises a plurality of phase lines, and wherein theapparatus for coupling signals is coupled to one of the phase lines. 11.An electricity distribution network according to claim 8 wherein thedistribution cable comprises a plurality of phase lines, and whereinthere is one apparatus for coupling signals coupled to each one of thephase lines, there also being a monitor for sensing the combination ofthe wanted and cancelling signals at a position on each of the phaselines and a single calculating means which controls the cancellingmeans, wherein there is a switch which is operable to selectivelyconnect the sensed signal from one of the monitors to the calculatingmeans.
 12. A method of coupling signals to a line, the methodcomprising: receiving a wanted signal at an input; coupling the wantedsignal onto the line at a first position; deriving, at a cancellingmeans, which is coupled to the input, a cancelling signal from thewanted signal, the cancelling means being operable to phase-shift thewanted signal; coupling the cancelling signal onto the line at a secondposition, spaced from the first position; and wherein the combination ofthe phase-shift and Propagation delays experienced by the signals isarranged such that the wanted signal and the cancelling signaldestructively combine in a single direction of propagation along theline while enabling propagation of the wanted signal along the line inthe other direction.
 13. A method according to claim 12 wherein the linecomprises a power line and the wanted signal comprises atelecommunications signal.
 14. A method of coupling communicationssignals on to an electricity distribution network comprising asubstation serving a plurality of distribution lines, the methodcomprising: coupling communications signals occupying a frequency bandto one of the lines by coupling a wanted communications signal onto theline at a first position and coupling a cancelling signal onto a line ata second position, spaced from the first position such that the wantedcommunications signal and cancelling signal destructively combine in adirection of propagation towards the substation; and, reusing thefrequency band for coupling different communications signals on toanother one of the plurality of lines.
 15. Apparatus for couplingtelecommunications signals to a power line, the apparatus comprising: afirst means for coupling a wanted telecommunications signal onto thepower line at a first position; a second means for coupling a cancellingsignal onto the power line at a second position, spaced from the firstposition; wherein the wanted telecommunications signal and thecancelling signal are arranged to destructively combine in a singledirection of propagation along the power line while enabling the wantedsignal to propagate in the other direction along the power line.
 16. Amethod of coupling telecommunications signals to a power line, themethod comprising: coupling a wanted telecommunications signal onto thepower line at a first position; coupling a cancelling signal onto thepower line at a second position, spaced from the first position; andwherein the wanted telecommunications signal and the cancelling signalare arranged to destructively combine in a single direction ofpropagation along the power line while enabling propagation of thewanted signal along the power line in the other direction.